Suppression of localized metal precipitate formation and corresponding metallization depletion in semiconductor processing

ABSTRACT

A structure for suppressing localized metal precipitate formation (LMPF) in semiconductor processing. For each metal wire that is exposed to the manufacturing environment and is electrically coupled to an N region, at least one P+ region is formed electrically coupled to the same metal wire. As a result, few excess electrons are available to combine with metal ions to form localized metal precipitate at the metal wire. A monitoring ramp terminal can be formed around and electrically disconnected from the metal wire. By applying a voltage difference to the metal wire and the monitoring ramp terminal and measuring the resulting current flowing through the metal wire and the monitoring ramp terminal, it can be determined whether localized metal precipitate is formed at the metal wire.

This application is a divisional application claiming priority to Ser.No. 11/550,853, filed Oct. 19, 2006; which is a divisional applicationof U.S. Pat. No. 7,173,338, issued Feb. 6, 2007.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to Localized Metal Precipitate Formation(hereafter referred to as LMPF) and the corresponding metallizationdepletion, and more particularly, to methods for suppressing andmonitoring LMPF, and suppressing interconnect depletion in semiconductorprocessing.

2. Related Art

During the process of fabricating chips on a semiconductor wafer, theremay be at least two electrically isolated metal wires exposed to themanufacturing environment at the surface of the wafer. Due to photoand/or other forms of excitation, one of the two metal wires (hereafterreferred to as the first wire) may be at a higher electrical potentialthan the other metal wire (hereafter referred to as the second wire).When the wafer undergoes a CMP (Chemical Mechanical Polishing) step,during which the wafer is exposed to an ionic solution, some metal atoms(M) of the first wire dissolve in the solution and become ionized(M->M^(n+)). As a result of the electrical potential difference betweenthe first and second wires, some of these ionized atoms move in thesolution from the first wire towards the second wire, and redeposit aslocalized metal precipitates (LMP) at the second wire (M^(n+)→M). TheseLMPs may cause a short-circuit in the devices on the wafer, which woulddegrade or destroy the ability of these devices to perform theirintended function. Also, in some situations, the metal ions thatdissolve into the ionic solution are drawn from a very small area of thefirst metal wire, resulting in localized removal of metal in that wire,often referred to as depletion or erosion of the metal. This localizedloss of material from the first wire degrades the current carryingcapability of the line and can lead to premature metallization failure.

Therefore, there is a need for a structure to prevent such localizedmetal precipitate formation and the corresponding metallizationdepletion. Also, there is a need for a method for forming the structure.There is also a need for a method for monitoring such LMPF.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor structure, comprising (a)an electrically conducting wire; and (b) first and second semiconductorregions being electrically coupled to the electrically conducting wireand being doped with first and second doping types, respectively,wherein the first and second doping types are of opposite doping types,wherein without the second semiconductor region, in response to thefirst semiconductor region being excited and the electrically conductingwire being directly exposed to an ionic solution, a first current flowsbetween the ionic solution and the first semiconductor region throughthe electrically conducting wire, and wherein with the presence of thesecond semiconductor region, in response to the first semiconductorregion being excited and the electrically conducting wire being directlyexposed to the ionic solution, a second current flows between the firstsemiconductor region and the second semiconductor region so as to reducethe magnitude of the first current.

The present invention also provides a method for fabricating asemiconductor structure, the method comprising (a) providing in thestructure an electrically conducting wire, and first and secondsemiconductor regions being electrically coupled to the electricallyconducting wire and being doped with first and second doping types,respectively, wherein the first and second doping types are of oppositedoping types, and wherein without the second semiconductor region, inresponse to the first semiconductor region being excited and theelectrically conducting wire being directly exposed to an ionicsolution, a first current flows between the ionic solution and the firstsemiconductor region through the electrically conducting wire; and (b)in response to the first semiconductor region being excited and theelectrically conducting wire being directly exposed to the ionicsolution, generating a second current between the first semiconductorregion and the second semiconductor region so as to reduce the magnitudeof the first current.

The present invention also provides a semiconductor structure,comprising (a) a first semiconductor region doped with a first dopingtype; and (b) a first photo-blocking layer covered on top of the firstsemiconductor region and adapted for reducing light reaching andexciting the first semiconductor region.

The present invention also provides a method for identifying LMPF(Localized Metal Precipitate Formation) in a semiconductor structure,the method comprising (i) providing in the semiconductor structure (a) afirst electrically conducting wire, (b) a first semiconductor regiondoped with N type dopants and electrically coupled to the firstelectrically conducting wire, (c) a monitoring ramp terminal being nearthe first electrically conducting wire, electrically disconnected fromthe first electrically conducting wire, and exposed to the atmosphere,and (d) a second electrically conducting wire; (ii) applying a voltagedifference to the monitoring ramp terminal and a select wire selectedfrom the group consisting of the first and second electricallyconducting wires; and (iii) measuring a current flowing between theselect wire and the monitoring ramp terminal so as to determine whetherLMPF occurs at the first electrically conducting wire.

The present invention also provides a method for determining thesensitivity degree of a fabrication process performed on a wafer, themethod comprising the steps of (a) providing in the wafer a plurality ofstructures having various LMPF likelihood ratios; (b) monitoring LMPF ateach of the plurality of structures and collecting monitoring data; and(c) determining the sensitivity degree of the fabrication process

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate cross-sectional views of different semiconductorstructures for suppressing LMPF and the corresponding metallizationdepletion, in accordance with embodiments of the present invention.

FIG. 2 illustrates a cross-sectional view of another semiconductorstructure for suppressing LMPF and the corresponding metallizationdepletion, in accordance with embodiments of the present invention.

FIGS. 3A-3B illustrate cross-sectional and top views, respectively, ofyet another semiconductor structure for monitoring LMPF, in accordancewith embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A illustrates a cross-sectional view of a semiconductor structure100 a, in accordance with embodiments of the present invention.

In one embodiment, the structure 100 a can be formed by first implantingan N well 120 in a P type silicon substrate 110. Next, two P+ regions124 a and 122 and an N+ region 126 are formed by ion implantation, withthe regions 124 a and 126 in N well 120.

Dielectric layer 130 is then deposited upon the entire structure andvias 132, 134 a, and 136 are formed in the dielectric layer 130 andfilled with a conducting material such as copper, tungsten, or any othersuitable metal. The vias 132, 134 a, and 136 are in direct physicalcontact with P+ regions 122 and 124 a, and N+ region 126, respectively.The N+ region 126 and the P+ region 122 constitute contacts to the Nwell 120 and the substrate 110, respectively. Excess metal from thedeposition process is removed by CMP. Then, dielectric layer 140 isdeposited upon the entire structure. Copper (Cu) wires 142 and 144 arethen formed in the dielectric layer 140. The copper wire 142 is indirect physical contact with the via 132, and the copper wire 144 is indirect physical contact with the vias 134 a and 136.

The formation of wires 142 and 144, as herein described and as shown instructure 100 a, is accomplished by a single damascene process. Inanother embodiment, not shown in the figures, wires 142 and 144, alongwith their corresponding vias 132, 134 a, and 136, can all be formedusing a dual damascene (dd) process. In the dd process, dielectriclayers 130 and 140 would be deposited together, then vias 132, 134 a,and 136, and trenches 142 and 144, are etched (either vias first, thentrenches, or in reverse order), and then both vias and trenches arefilled at the same time before the CMP step is performed to removeexcess metal.

In order to understand how LMPF is suppressed in the structure 100 a,assume that the P+ region 124 a and the via 134 a were not formed in theabove-described steps. Under photo excitation (i.e., the transfer ofphoto energy into the structure 100 a) and/or other forms of excitationof the structure 100 a, free holes (positive charges) and electrons arecreated in the P substrate region 110 and N well region 120. Throughdiffusion and field drift, the free holes can move toward the P+ region122, and the free electrons can move toward the N+ region 126. As aresult, the P+ region 122 (a contact to the P substrate region 110) andN+ region 126 (a contact to the N well 120) become anode and cathode,respectively, of a voltage cell (not shown) with an establishedelectrical potential difference between the two regions 122 and 126.

Assume further that the structure 100 a undergoes a CMP (ChemicalMechanical Polishing) step or any other process that exposes thestructure 100 to an ionic or electrolytic solution. As a result, anionic solution 150 of the CMP is applied to the top surface 148 of thestructure 100 a. The surface where fabrication processes are directed isthe top surface of the structure or wafer. With the P+ region 122 andthe N+ region 126 being the anode and cathode of the voltage cell,respectively, a current flows from the anode 122 through the via 132,the Cu wire 142, the solution 150, the Cu wire 144, and the via 136 tothe cathode 126.

In the solution 150, the current comprises copper ions Cu+ and Cu++dissolved into the solution 150 from the Cu wire 142 and moving in thesolution 150 towards the Cu wire 144. At the Cu wire 144, the copperions combine with free electrons created in the N well 120 and moving upto the Cu wire 144 through the N+ region 126 and the via 136 (i.e., path127) so as to form copper localized metal precipitate (LMP) 152 at theCu wire 144. As a result of some copper atoms of the copper wire 142being dissolved into the solution 150 as copper ions, the copper wire142 is depleted of material (i.e., metallization depletion).

The above occurrence describes what would happen without the presence ofthe P+ region 124 a and the via 134 a. Now, with the presence of P+region 124 a and the via 134 a, most of the excess free electronscreated in the N well 120 by the excitation mechanisms mentioned above(photo and/or others) will flow from the N+ region 126 through the via136, the Cu wire 144, and the via 134 a to the P+ region 124 a (i.e.,path 128 a). As a result, fewer free electrons from N well region 120flow along the path 127 and become available to combine with the copperions in the solution 150 to form copper LMP 152. In other words, copperLMPF is suppressed.

With fewer free electrons from N well region 120 flowing along the path127, the current flowing in the solution 150 from the Cu wire 142 to theCu wire 144 is reduced. As a result, fewer copper atoms of the Cu wire142 dissolve into the solution 150 as copper ions. In other words,metallization depletion at the Cu wire 142 is also suppressed.

FIG. 1B illustrates a cross-sectional view of another semiconductorstructure 100 b, in accordance with embodiments of the presentinvention. The structure 100 b is similar to the structure 100 a of FIG.1A, except that the via 134 a is omitted and a conducting strap 129 b isformed electrically connecting the P+ region 124 a and the N+ region126. As a result, excess free electrons from N well 120, created by theexcitation mechanisms mentioned above (photo and/or others), will flowfrom the N+ region 126 through the conducting strap 129 b to the P+region 124 a (i.e., via the path 128 b). As a result, substantiallyfewer free electrons from N well regions 120 flow along the path 127 andbecome available to combine with the copper ions in the solution 150 toform the copper LMP 152. In other words, copper LMPF is suppressed.

In one embodiment, the conducting strap 129 b may comprise a localinterconnect material, such as tungsten silicide, cobalt silicide,nickel silicide, or any other suitable interconnect material. If theconducting strap 129 b is made of cobalt silicide, for example, it canbe formed by depositing a layer of cobalt directly above the P+ region124 a and the N+ region 126. Then, prior to the deposition of dielectriclayer 130, the structure 100 b is heated up so as to make the cobaltlayer react with silicon material of the N well 120 and form the cobaltsilicide conducting strap 129 b.

With fewer free electrons from N well region 120 flowing along the path127, the current flowing in the solution 150 from the Cu wire 142 to theCu wire 144 is reduced. As a result, fewer copper atoms of the Cu wire142 dissolve into the solution 150 as copper ions. In other words,metallization depletion at the Cu wire 142 is also suppressed.

FIG. 1C illustrates a cross-sectional view of another semiconductorstructure 100 c, in accordance with embodiments of the presentinvention. The structure 100 c is similar to the structure 100 b of FIG.1B, except that the conducting strap 129 b is omitted and the P+ region124 a (now referred to as P+ region 124 c) are formed abutting (i.e., inphysical contact with) the N+ region 126 forming a p-n junction 124c,126.

As a result, most of the free electrons from N well region 120, createdby the excitation mechanisms mentioned above (photo and/or others) willforward bias the p-n junction 124 c, 126 causing the excess electrons toflow from the N+ region 126 directly to the P+ region 124 c (i.e., viathe path 128 c). As a result, fewer free electrons from N Well regions120 flow along the path 127 and become available to combine with thecopper ions in the solution 150 to form copper LMP 152. In other words,copper LMPF is suppressed.

With fewer free electrons from N well region 120 flowing along the path127, the current flowing in the solution 150 from the Cu wire 142 to theCu wire 144 is reduced. As a result, fewer copper atoms of the Cu wire142 dissolve into the solution 150 as copper ions. In other words,metallization depletion at the Cu wire 142 is also suppressed.

FIG. 1D illustrates a cross-sectional view of another semiconductorstructure 100 d, in accordance with embodiments of the presentinvention. The structure 100 d is similar to the structure 100 a of FIG.1A, except that the P+ region 124 a (now referred to as P+ region 124 d)is formed outside the N well 120.

As a result, some of the free electrons from N well region 120, createdby the excitation mechanisms mentioned above (photo and/or others) willflow from the N+ region 126 through the via 136, the Cu wire 144, andthe via 134 a to the P+ region 124 d (i.e., via the path 128 d). As aresult, fewer free electrons from N well regions 120 flow along the path127 and become available to combine with the copper ions in the solution150 to form copper LMP 152. In other words, copper LMPF is suppressed.In this configuration, the N well 120 is effectively electricallyshorted to the substrate 110.

With fewer free electrons from N well region 120 flowing along the path127, the current flowing in the solution 150 from the Cu wire 142 to theCu wire 144 is reduced. As a result, fewer copper atoms of the Cu wire142 dissolve into the solution 150 as copper ions. In other words,metallization depletion at the Cu wire 142 is also suppressed.

FIG. 1E illustrates a cross-sectional view of another semiconductorstructure 100 e, in accordance with embodiments of the presentinvention. The structure 100 e is similar to the structure 100 a of FIG.1A, except that the P+ region 124 a and the via 134 a are omitted, andthat photo-blocking layers 129 a and 129 b are formed directly on the P+region 122 and the N well 120, respectively.

In one embodiment, the photo-blocking layers 129 a and 129 b comprisesilicide such as cobalt silicide which is capable of preventing light(or in general, electromagnetic waves) from reaching the N well region120 to create free electrons. More specifically, with the presence ofthe photo-blocking layer 129 b, fewer free electrons are generated toflow along the path 127 and become available to combine with the copperions in the solution 150 to form copper LMP 152. In other words, copperLMPF is suppressed.

With fewer free electrons from N well region 120 flowing along the path127, the current flowing in the solution 150 from the Cu wire 142 to theCu wire 144 is reduced. As a result, fewer copper atoms of the Cu wire142 dissolve into the solution 150 as copper ions. In other words,metallization depletion at the Cu wire 142 is also suppressed.

In one embodiment, photo-blocking layers 129 a and 129 b can be formedby depositing layers of metal (such as cobalt) directly above the P+region 122 and the N well 120. Then, the structure 100 e is heated up soas to make the cobalt layers react with silicon material of the P+region 122 and the N well 120 so as to form the photo-blocking layers129 a and 129 b. Subsequent to the high temperature annealing process,un-reacted metal may be removed from the wafer in a suitable wet etchingprocess.

FIG. 2 illustrates a cross-sectional view of yet another semiconductorstructure 200, in accordance with embodiments of the present invention.The structure 200 is similar to the structure 100 a, except that N+regions become P+ regions, and P+ regions become N+ regions. Forinstance, the P+ region 122 of FIG. 1A becomes the N+ region 222 of FIG.2. In one embodiment, the element 210 may be an n-type substrate. Inanother embodiment, the element 210 is a large N well region (not shown)embedded in a p-type substrate (not shown).

In order to understand how LMPF is suppressed in the structure 200,assume that the N+ region 224 a and the via 234 were omitted in thestructure 200. Under photo excitation and/or other forms of excitationof the structure 200, free electrons and holes are created in the Nregion 210 and P well region 220, respectively. As a result, the N+region 222 and the P+ region 226 become cathode and anode, respectively,of a voltage cell (not shown).

Assume further that the structure 200 undergoes a CMP (ChemicalMechanical Polishing) step or any other process that exposes thestructure to an ionic or electrolytic solution. As a result, an ionicsolution 250 is present at the top surface 248 of the structure 200.With the N+ region 222 and the P+ region 226 being the cathode and anodeof the voltage cell, respectively, a current flows from the anode 226through the via 236, the Cu wire 244, the solution 250, the Cu wire 242,and the via 232 to the cathode 222.

In the solution 250, the current comprises copper ions Cu+ and Cu++dissolved into the solution 250 from the Cu wire 244 and moving in thesolution 250 towards the Cu wire 242. At the Cu wire 242, the copperions combine with the free electrons created in the N region 210 andmoving up to the Cu wire 242 through the N+ region 222 and the via 232(i.e., via the path 227) so as to form copper LMP 252 at the Cu wire242. As a result of some copper atoms of the copper wire 244 beingdissolved into the solution 250 as copper ions, the copper wire 244 isdepleted of material (i.e., metallization depletion).

The above occurrence describes what would happen without the presence ofN+ region 224 a and the via 234. Now, with the presence of the N+ region224 a and the via 234, excess free electrons are created in the N+region 224 a and flow through the via 234, the Cu wire 244, and the via236 to the P+ region 226 (i.e., via the path 228 a). As a result, theconcentration of excess free holes in the P well region 220 aresubstantially reduced, and therefore the potential difference betweenthe P+ region 226 and the N+ region 222 are reduced. As a result, thenumber of free electrons flowing through the path 227 is substantiallyreduced. Therefore, fewer free electrons become available at the Cu wire242 to combine with the copper ions in the solution 250 to form copperLMP 252. In other words, copper LMPF is suppressed.

With fewer free electrons from N+ region 222 flowing along the path 227,the current flowing in the solution 250 from the Cu wire 244 to the Cuwire 242 is reduced. As a result, fewer copper atoms of the Cu wire 244dissolve into the solution 250 as copper ions. In other words,metallization depletion at the Cu wire 244 is also suppressed.

Other embodiments of the present invention can be obtained by replacingP type regions by N type regions and replacing N type regions by P typeregions in the structures 100 b, 100 c, 100 d, and 100 e.

In summary, for each N well (like the N well 120 of FIGS. 1A-1D) in asemiconductor structure which is electrically connected to a metal wire,at least one P+ region (P doping type is the opposite doping type of theN doping type), like the P+ regions 124 a-124 d of FIGS. 1A-1D,respectively, can be formed electrically connected to the sameconducting wire. Without this P+ region, and in response to theexcitation mechanisms acting upon the N well and substrate regions, andthe metal wire being directly exposed to an ionic solution, freeelectrons (negative charges) would move from the N well to the surfaceof the metal wire in contact with the ionic solution, attracting andcombining with Cu+ and Cu++ ions, which then aggregate locally, formingone or more precipitates at that metal wire. However, with the presenceof this P+ region, in response to the excitation mechanisms acting uponthe N well and the metal wire being directly exposed to the ionicsolution, free electrons, which would otherwise be available to reactwith metal ions in the ionic solution, are contained within the solidsemiconductor material. As a result, excess negative charges that wouldaccumulate at the surface of the metal wire connected to the N well (andin contact with the ionic solution) are reduced or eliminated, and feweror no Cu+ or Cu++ ions aggregate at the surface of the metal wire.Therefore, localized metal precipitate formation (LMPF) at the metalwire connected to the N well is suppressed. This is illustrated in FIGS.1A-1D.

Similarly, for each P well (like the P well 220 of FIG. 2) in asemiconductor structure that is electrically connected to a conductingwire, at least one N+ region (like the N+ region 224 a of FIG. 2) can beformed, electrically connected to the same conducting wire. Without thisN+ region, in response to the excitation mechanisms acting upon the Pwell, and the conducting wire contacting it through the P+ region beingdirectly exposed to an ionic solution, excess free holes (positivecharges) would move from the P well to the surface of the conductingwire in contact with the ionic solution, where they produce copper ions,enabling a current to flow through the ionic solution from the P+ regionto another conducting wire exposed to the solution and connected toanother N+ region. As a result of this current, LMP may form at theother conducting wire that is directly exposed to the ionic solution andelectrically coupled to the other N+ region. However, with the presenceof this N+ region, in response to the excitation mechanisms acting uponthe P well regions and the conducting wire being directly exposed to theionic solution, free holes which would otherwise be available to producemetal ions in the ionic solution, are contained within the solidsemiconductor material. As a result, excess positive charges that wouldmove to the surface of the wire connected to the P well, and in contactwith the ionic solution, are reduced or eliminated. Therefore, thecurrent flowing through the ionic solution to the N+ region is reducedor eliminated and LMPF at the conducting wire connected to the N+ regionis suppressed. This is illustrated in FIG. 2.

FIGS. 3A-3B illustrate cross-sectional and top views, respectively, ofyet another semiconductor structure 300 for monitoring LMPF, inaccordance with embodiments of the present invention. The structure 300is similar to the structure 100 a of FIG. 1A, except that the P+ region124 a and the via 134 a are omitted and that a monitoring ramp terminal348 is formed around and electrically disconnected from the Cu wire 144.This is intentionally done to exacerbate LMPF. In one embodiment, themonitoring ramp terminal 348 may comprise copper or any conductingmaterial.

In one embodiment, a voltage difference is applied to the monitoringramp terminal 348 and the Cu wire 144. If there is no LMP formed at theCu wire 144 (like the LMP 152), the resulting current flowing betweenthe monitoring ramp terminal 348 and the Cu wire 144 should be low (˜pA)due only to leakage current. If there is LPM formed at the Cu wire 144(like the LMP 152), the resulting current flowing between the monitoringramp terminal 348 and the Cu wire 144 through the LMP 152 will be high(˜μA or higher). As a result, by applying a voltage difference to themonitoring ramp terminal 348 and the Cu wire 144 and measuring theresulting current flowing between the two terminals 348 and 144, it canbe determined whether LMPF exists at the Cu wire 144. In one embodiment,if the measured current flowing between the two terminals 348 and 144exceeds a pre-specified value, it is determined that LMP is formed atthe terminal 144 and the structure 300 has identified a processsensitivity to LMPF.

In an alternative embodiment, a voltage difference is applied to themonitoring ramp terminal 348 and the Cu wire 142 (as opposed to the Cuwire 144). If there is no LMP formed at the Cu wire 144 (like the LMP152), the resulting current flowing between the monitoring ramp terminal348 and the Cu wire 142 should be low (˜μA) due only to current leakage.If there is LMP formed at the Cu wire 144 (like the LMP 152), theresulting current flowing between the monitoring ramp terminal 348 andthe Cu wire 142 will be high (˜μA or higher). One possible current pathmay be from the Cu wire 142 through the via 132, the P+ region 122, theN well 120, the N+ region 126, the via 136, the Cu wire 144, and the LMP152 to the monitoring ramp terminal 348 (i.e., path 307), assuming thevoltage of the Cu wire 142 is higher than that of the monitoring rampterminal 348. As a result, by applying a voltage difference to themonitoring ramp terminal 348 and the Cu wire 142 and measuring theresulting current flowing between the two terminals 348 and 142, it canbe determined whether LMPF exists at the Cu wire 144, and the processsensitivity to LMPF is determined.

For the structure 300, the monitoring ramp terminal 348 is formed aroundand electrically disconnected from the Cu wire 144 where LMP may beformed. LMP may be formed at the Cu wire 144 because the Cu wire 144 isexposed to an ionic solution and is electrically coupled to a N region(the N well 120). In general, a monitoring ramp terminal like themonitoring ramp terminal 348 may be formed around and electricallydisconnected from any conducting wire which is exposed to an ionicsolution and is electrically coupled to a N region.

In one embodiment, for the structure 300, a monitoring terminal like theregions 142,132,122 can be specially formed for monitoring LMPF at thewire 144. In another embodiment, any ground pad near the rampingterminal 348 can be used as the monitoring terminal for monitoring LMPFat the wire 144.

For the structure 200 of FIG. 2, a monitoring ramp terminal (not shown)similar to the monitoring ramp terminal 348 of FIG. 3 may be formedaround and electrically disconnected from the Cu wire 242, where copperLMP may deposit. Then, by applying a voltage difference to themonitoring ramp terminal and the Cu wire 242 (or the Cu wire 244) andmeasuring the resulting current flowing between the two terminals, itcan be determined whether LMPF exists at the Cu wire 242. In oneembodiment, if the measured current flowing between the two terminals348 and 242 exceeds a pre-specified value, it is determined that LMP isformed at the terminal 242 and the structure 200 has identified aprocess sensitivity to LMPF.

With reference to FIG. 3, it has been shown that the likelihood of LMPFoccurring at the Cu wire 144 of the structure 300 depends on an LMPFlikelihood ratio of a first area of the N well 120 to a second area ofthe Cu wire 144. Increasing this ratio creates more free electronsduring excitation of the N well 120 allowing more Cu ions to combinewith the free electrons and accumulate on Cu wire 144. This means thatthe higher the LMPF likelihood ratio is, the more sensitive (i.e.,prone) to LMPF the structure 300 becomes.

In one embodiment, multiple structures similar to the structure 300 ofFIG. 3 (hereafter, referred to as the structures 300) are formed on awafer (not shown). The multiple structures 300 have various LMPFlikelihood ratios, for example, 100:1, 250:1, 300:1, 750:1, etc. Then,during fabrication processes performed on the wafer, LMPF is monitoredat the Cu wires 144 of these multiple structures 300. Based on themonitoring data, the fabrication process sensitivity to LMPF can bedetermined.

For example, assume that no LMPF is detected at the structures 300having the LMPF likelihood ratios of 250:1 or lower, and that LMPF isdetected at the structures 300 having the LMPF likelihood ratios of300:1 or higher. As a result, it can be said that the particularfabrication process is sensitive to LMPF for LMPF likelihood ratios of300:1 or higher. From this information, the design can be changed so asto avoid damages resulting from LMPF by making sure that all LMPFlikelihood ratios through out the wafer are 250:1 or lower. The LMPFlikelihood ratio of 300:1 can be referred to as the sensitivity degreeof the fabrication process. Typically, LMPF likelihood ratios of thestructures 300 lie between, but are not limited to, the range of 100:1and 750:1.

While above discussions focus on the suppression of LMPF at the cathode,the similar arguments also apply to the anode side. If the currentbetween the cathode and the anode is reduced, the metal depletion at theanode will also be suppressed.

In the embodiments described above, copper wires are used. In general,any conducting material can be used.

In the embodiments described above, silicide materials are used. Ingeneral, any materials that can reduce the light reaching and excitingthe n-doped and p-doped regions can be used.

While particular embodiments of the present invention have beendescribed herein for purposes of illustration, many modifications andchanges will become apparent to those skilled in the art. Accordingly,the appended claims are intended to encompass all such modifications andchanges as fall within the true spirit and scope of this invention.

1. A semiconductor structure, comprising: a first semiconductor regiondoped with a first doping type; and a first photo-blocking layer coveredon top of the first semiconductor region and adapted for reducing lightreaching and exciting the first semiconductor region; a secondsemiconductor region doped with a second doping type, the first andsecond doping types being of opposite doping types; and a secondphoto-blocking layer covered on top of the second semiconductor regionand adapted for reducing light reaching and exciting the secondsemiconductor region; and an ionic solution electrically coupled to boththe first and second semiconductor regions.
 2. The structure of claim 1,further comprising an electrically conductive wire electrically couplingthe first semiconductor region to the ionic solution.
 3. The structureof claim 2, wherein the ionic solution comprises copper ions, andwherein the electrically conductive wire comprises copper.
 4. Thestructure of claim 1, wherein the first and second doping types areP-type and N-type, respectively.
 5. The structure of claim 1, whereinthe first and second photo-blocking layers comprise a silicide material.6. The structure of claim 5, wherein the silicide material comprisescobalt silicide.